The domain is video compression. We are particularly interested in block-based compression schemes, realizing an adaptive frame/field coding at a picture or block level, a coding providing a substantial gain in compression. This is for example the MPEG4 part-10 (H264) standard. In these classically implemented diagrams, the frame/field coding decision at the macroblock level is taken within the coding loop, introducing a double complexity with respect to a non-adaptive encoder that only operates in frame mode or field mode. This functionality is costly in terms of realization.
This coding decision depends, amongst other elements, on the temporal correlation and as a result, the motion estimations made by the coding circuit for the predictive coding, generally of the block matching type and of the hierarchical type.
For example in the case of a “block-matching” type hierarchical motion estimator for an encoder compatible with the H264 or MPEG4 part 10 standard, a motion vector field is calculated for each block size (4×4, 4×8, 8×4, 8×8, 8×16, 16×8, 16×16) and for each reference picture. For coding by macroblock pair, known as MBAFF, the English acronym for MacroBlock Adaptive Frame Field, the estimator provides, per macroblock, 1 motion vector field for the frame, 2 motion vector fields for the fields. Hence, for 16×16 size macroblocks, 41×3 motion vectors may be calculated for each reference picture, this number being multiplied if several reference pictures are used, such as in multi-reference mode. The choice amongst this multiplicity of coding possibilities is very costly in terms of calculations.
Secondly, in the example of the H264 standard, the decision as regards frame or field coding may be made independently for each vertical macroblock pair in the picture, at size 16×32 for the luminance block. This coding option is known as MBAFF, the acronym for “macroblock-adaptive frame/field coding”. For a macroblock pair coded in frame mode, each macroblock contains the lines of the interlaced picture. For a macroblock pair coded in field mode, the upper macroblock contains the lines of the upper field or odd field, and the lower macroblock contains the lines of the lower field or even field.
The coding mode decision, which may be based on the measurement of coding costs and the distortion associated with the mode, is made at the level of the macroblock pair and not at the macroblock level as for the MPEG2 standard, hence multiplying the possible combinations and thus the calculation load.
One solution to reduce the calculations consists in choosing between fields and frames for low resolution pictures, to limit the calculations at higher resolutions. These premature choices, made at the upper levels of the pyramid and hence at lower resolutions than those of source pictures, do not provide good results, due to the insufficient vertical definition, an upper field, lower field selection corresponding to a shift in one line.